1. Field of the Invention
This invention relates to a length measuring system comprising a scale member and a scanning unit, which is movable along said scale member, wherein the scale member is accommodated in a scale carrier, which constitutes a tubular protective housing for the scale member and the scanning unit, and is connected to said scale carrier by means which permit a relative longitudinal movement. The scale carrier is provided with fasteners, such as screws, for connection to an object, such as a bed of a machine, at least at locations adjacent to end blocks of the scale carrier and, optionally, at a plurality of locations spaced along the scale carrier, and compensating means adjacent to these locations serve to effect a compensation of length differences resulting from a differential thermal expansion of the object and the scale carrier.
Such design features are embodied in length measuring systems to ensure that the scale member will not be acted upon by undesired forces and/or deformations caused by thermal expansion and that a displacement of a reference point of the scale, which is provided at one end or at the center of the scale and in incremental measuring systems may be defined, e.g., by a reference mark with reference to an associated reference point on the object, e.g., a zero measurement location of a machine tool, should be avoided as far as possible.
2. Prior Art
From DE-A 17 37 403 it is known that the scale carrier can be secured to the machine bed by screws, which are inserted through larger-diameter bores in the scale carrier, and the clearance between the diameters is occupied by bushings, which are made of rubber-elastic material and extend in the bores and surround the screws. But in that design the scale carrier and the measuring system can be displaced transversely to its longitudinal direction and for this reason different compensating means are preferred. According to U.S. Pat. No. 3,816,002 at least one end of the scale carrier is formed with a slot for receiving a screw fastener, which in the slot is enlarged to form a locating pin and by means of a coil spring or a series of disk springs the head of said screw applies pressure to the longitudinal edges of the slot so that the scale carrier can expand and contract along the slot under the action of heat. From De-A 26 43 304 it is known that the scale member, which in most cases is made of glass, can be secured to the scale carrier by means of a highly elastic adhesive in such a manner that a flat surface of the scale member lies on the thin adhesive coating and can be fixed adjacent to a reference point by means of an adhesive having a low elasticity. It is known from DE-A 28 53 771 that a tubular scale carrier may be provided with end blocks, which are formed with through holes for screw fasteners and which are not rigidly connected directly to the respective ends of the scale carrier and that each of said end blocks at its end facing the scale carrier may be formed with meandering slots extending from the top and bottom, respectively, so that weak portions are provided, which permit a longitudinal displacement of the scale carrier relative to the mounting blocks, which are rigidly mounted. That design has the disadvantage that the weak portions require additional space in the longitudinal direction of the scale member, a satisfactory guidance of the scale carrier only in its longitudinal direction is not ensured, and the end blocks cannot be provided with plugs, which extend into and are sealed in the interior of the tubular scale carrier so that said interior is hermetically sealed at both ends. Means for a length compensation provided at end-mounting elements may alternatively consist of additional spring elements, spring hinges, resiliently interconnected angle members, and hinges, which pivotally connect the carrier to each mounting block on an axis which is at right angles to the longitudinal direction of the scale carrier. In all said cases there is a risk that the overall arrangement may be unstable or that the longitudinal guidance of the scale member relative to the object is not sufficiently precise or, in many cases, that the length of the overall arrangement is highly undesirably increased by the additional compensating means provided adjacent to the ends, or in that a temperature rise and temperature drop will result in undesired motions. Particularly if the scale member is long the known continuous fixation by means of a thin and yieldable adhesive layer will not ensure a sufficient compensation of length differences between the scale member and the scale carrier so that the forces which are due to thermal expansion will result in an undesired deformation and/or stressing of the scale member if its length exceeds a certain limit and in any case if its length exceeds an order of 1 meter.